There’s a ton of old AT keyboards making their way to the landfill. We’ll show you how to recycle one as an input device for your next project.

Connection

Bus Pirate

PC AT keyboard (pin #)

SDA

KBD Data (3)

SCL

KBD Clock (1)

+5volts

VDD (5)

GND

GND (2)

AT keyboards communicate over a bidirectional two-wire interface. The bus is open collector, but keyboards already have internal pull-up resistors. The PC AT keyboard protocol is described here. We used our Bus Pirate tool to demonstrate the keyboard protocol, but the same basic principals apply to any microcontroller.

We connected the Bus Pirate to the keyboard as outlined in the table. We believe that this is a through-hole female AT keyboard jack, but we haven’t tested it. Do you know of a source for new sockets?

Protocol

The keyboard provides the clock signal for all data transfers; the PC side resembles a slave device. None of the existing Bus Pirate interface libraries work with an external clock, so we wrote a simple AT keyboard decoder library. The library depends on the keyboard’s clock signal, and it’ll hang if the keyboard fails or isn’t connected. If you use our library in your own project, consider adding a timeout delay in the readbit() and writebit() functions.

PC to keyboard command codes

Code

Command

0xed

Set status LEDs

0xee

Echo 0xee

0xf0

Set scancode type

0xf3

Set repeat rate

0xf4

Keyboard enable

0xf5

Keyboard disable

0xfe

Resend last byte

0xff

Reset keyboard

A PC uses these commands to control various functions of an AT keyboard. The keyboard responds to commands with an acknowledge byte (oxfa). In our experience, the keyboard will reset if the response byte is not read shortly after the command is sent.

Keyboard to PC response codes

Code

Response

0xfa

Acknowledge

0xaa

Self test passed

0xee

Echo response

0xfe

Resend last byte

0x00 or 0xff

Error or buffer overflow

The keyboard has a number of single byte response codes. Most PC commands are acknowledged with 0xfa. 0xaa is sent after a keyboard reset.

The AT keyboard library follows the standard Bus Pirate syntax. Numeric values are sent to the keyboard as bytes, ‘r’ reads a byte from the keyboard. The protocol is clocked by the keyboard so bitwise operations are disabled. If no data is available, the read will return ‘NONE’.

We can test the connection to the AT keyboard using the echo command, 0xee. The keyboard will respond 0xee if our connections are correct.

The keyboard responds to commands with an ACK bit at the protocol level, and then again with an ACK byte. We found that our test keyboards reset automatically if the ACK byte wasn’t read immediately after sending the command.

The num, caps, and scroll lock LEDs are controlled by the 0xed command. The last three bits of a second byte (ob111) indicate which LEDs to light. It’s very important to perform all four byte operations within the keyboard timeout period, or the keyboard will reset.

A key press sends scancodes, multi-byte sequences that represent the key presses. In the example, we pressed space which has the scancode 0x29. When a key is released, the keyboard sends 0xf0 and the scancode for the key (0x29). Each key press results in a similar three part sequence.

This is just a simplified version of the previous example. Rather than read three bytes individually, we used the bulk read command. Again, we get the space scancode sequence. Our attempt to read a non-existant fourth byte fails.

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15 thoughts on “Parts: AT keyboard”

I actually hooked up an old IBM model M to my arduino last week. how coincidental. :)

As for sockets, what I did is take apart an old MIDI cable I didn’t need. MIDI uses the same physical connector (5-pin DIN), but it only uses three of the five pins, so you can’t just use those wires.
I stripped it down to the actual connector, which had stubs for all 5 pins, and connected wires to the right ones.

It was fun reading the bits into an 8 bit shiftregister on the Gameboy’s link port. Had to waste time in the interrupt to get the last 3 bits.
But then I realized such keyboard was way too big for portable usage.

I used a keyboard as input on my sr design project using an atmel based board (pre arduino). The really fun part about the keyboard is keeping the state of the caps/shift/numlock/scroll lock buttons and then talking back to the keyboard to turn those lights on and off…

to interface I desoldered a ps2 connector from a scrap mobo and put it on a proto board… then i could use any ps2 keyboard, they all use the same base protocol.

First off, some pictures, as promised:
my model M. It’s quite possibly older than I am.

An old midi-computer cable I had.
note: these pictures were taken after I had hacked it together. This cable used to have two MIDI female ends, as all MIDI cables do, one for in, one for out.
top:
front:

first, I chopped off one of the midi ends pretty high up, but discovered that there were only three wires inside! after checking, I found out that MIDI only uses three of the pins.
What I did then was chop off the cable at the base of the connector. Once that was done, the rubber jacket on the connector actually slid off quite easily, with a bit of force.
Inside that, there was the metal connector, and a blob of some sort of plastic on the end. The plastic was a bit more difficult to remove, but once I got it off, I had a simple 5-pin din socket, with connector stubs on the other end.

I wrapped and taped some wire on the end.
results:

top:
front:

and then that could be wired straight into an arduino, being at an electrically compatible level (I think).

Does anyone have any advice on how to use an AT keyboard as a regular input device on a USB only computer?
It looked like the PS/2 and AT protocols were pretty much the same, so could I wire in my keyboard to a ps/2 to usb adapter? What did other users do?

Also, what does hackaday have against capital letters in the comments?

I don’t suppose anyone has any experience using the bus pirate to hack a macbook keyboard? The cable coming from it has a 10 pin connector. Haven’t received the bus pirate yet, just wondering if someone can point me in the right direction first. Thanks